The excavation of rock is a primary activity in the mining, quarrying and civil construction industries. There are a number of important unmet needs of these industries relating to the excavation of rock and other hard materials. These include:
Reduced Cost of Rock Excavation PA1 Increased Rates of Excavation PA1 Improved Safety and Reduced Costs of Safety PA1 Better Control Over the Precision of the Excavation Process PA1 Cost Effective Method of Excavation Acceptable in Urban and Environmentally Sensitive Areas PA1 (a) releasing gas into the bottom of a hole located in a free surface of the hard material; PA1 (b) sealing the gas in the bottom of the hole to pressurize the hole bottom and cause a fracture to propagate from the bottom of the hole, thereby forming a fractured portion of the hard material a portion of which is exposed in the free surface surrounding the hole; and PA1 (c) impacting the fractured portion exposed at the free surface with an impact breaker to remove the material in the fractured portion from the free surface. The amount of blasting agent used to form the gas is typically relatively small. The fracture is an existing fracture that intercepts the hole bottom, the pressurized region of the hole, or a new fracture propagated from a bottom corner of the hole.
Drill & blast methods are the most commonly employed and most generally applicable means of rock excavation. These methods are not suitable for many urban environments because of regulatory restrictions. In production mining, drill and blast methods are fundamentally limited in production rates while in mine development and civil tunneling, drill and blast methods are fundamentally limited in advance rates because of the cyclical nature of the large-scale drill & blast process.
Tunnel boring machines are used for excavations requiring long, relatively straight tunnels with circular cross-sections. These machines are rarely used in mining operations.
Roadheader machines are used in mining and construction applications but are limited to moderately hard, non-abrasive rock formations.
Mechanical impact breakers are currently used as a means of breaking oversize rock, concrete and reinforced concrete structures. Mechanical impact breaker technology has advanced by increasing the blow energy and blow frequency of the impact tool through the use of high-energy hydraulic systems; and through the use of high-strength, high-fracture-toughness steels for the tool bit. Mechanical impact breakers can be used in almost any workplace setting because of the absence of air-blast and their relatively low seismic signature. As a general excavation tool, mechanical impact breakers are limited to relatively weak rock formations having a high degree of fracturing. In harder rock formations (unconfined compressive strengths above 60 to 80 MPa), the excavation effectiveness of mechanical impact breakers drops quickly and tool bit wear increases rapidly. Mechanical impact breakers cannot, by themselves, excavate an underground face in massive hard rock formations economically.
Small-charge blasting techniques can be used in all rock formations including massive, hard rock formations. Small-charge blasting includes methods where small amounts of blasting agents are consumed at any one time, as opposed to episodic conventional drill and blast operations which involve drilling multiple hole patterns, loading holes with explosive charges, blasting by millisecond timing the blast of each individual hole and in which tens to thousands of kilograms of blasting agent are used.
Small-charge blasting may produce flyrock which is unacceptable to nearby machinery and structures and may generate unacceptable air-blast and noise. In addition, small-charge blasting techniques cannot economically be used to excavate with the precision often required.
There is thus a need for a method and means to break rock efficiently and with low-velocity fly-rock such that drilling, mucking, haulage and ground support equipment can remain at the working face during rock breaking operations.